Mohamud apparently thought the city's Pioneer Courthouse Square would be the perfect location for the attack since it would be packed with people.
On Friday, Mohamud tried to detonate what he thought was a bomb-laden van near the Christmas tree-lighting ceremony by using a cellular phone detonator. When he dialed the phone, however, authorities moved in and arrested him. There were no injuries.
Officials said Mohamud had no formal ties to foreign terror groups, although he had reached out to suspected terrorists in Pakistan.
Mohamud is scheduled to appear in court on Monday, and it wasn't clear if he had a lawyer yet. He was charged with attempted use of a weapon of mass destruction, which carries a maximum sentence of life in prison.
He graduated from high school in Beaverton and dropped out of Oregon State University in Corvallis on Oct. 6 before declaring a major.
Sunday, a fire broke out at an Islamic center near the university. No injuries were reported and officials were investigating the origin of the blaze at Salman Alfarisi Islamic Center.
The Associated Press contributed to this report.